postgresql: foreign key to either tableA or tableB

I'm trying to figure out how to define the schema of a database on with Postgresql 8.

I have 2 tables:

Journals, Books

that define the publications I have

Journal:
id_j, name, issn, other fields

Book:
id_b, name, isbn, author, other fields

and I have another table Scans that logically refers both the previous tables.

Scans:
id, medium, source, status

each Journal or Book can have more than one Scan, but each Scan can refer only one Journal or Book.

To formalize this, my first idea was to put two foreign keys in Scans like

Scans:
id, medium, source, status, id_j, id_b

and fill in either id_j or id_b

but this solution seems to me a bit weird.

I don't want (if it is possible) to define the table in a way like this:

Scans:
id, medium, source, status, id_other_table, other_table_name

because I would like to have a formal connection between the tables.

Any idea?